Most pre-1970 Government Models came with the dark brown plastic grips with the molded-in Colt logo. What do the grips on the gun look like? Do they have metallic Colt medallions and, if so, what color are they. Or are they plain checkered wood?
Colt had a problem with the plastic grips. Some of them shrank badly and cracked around the grip screws. If your gun had the plastic grips initially and they developed this problem, they may have been replaced with wood grips.
What is the serial number of the pistol you are looking at? Does the serial number on the box match the gun? What is the finish, blue or nickel? What does the box look like?
These factors and more (particularly the condition of the gun) will dictate it's worth.